Of all the votes I cast during 34 years in the House of Representatives, the only one I really regret had to do with Vietnam, on Aug. 6, 1964. A couple of days earlier, the Maddox, an American destroyer on intelligence patrol in the Gulf of Tonkin off the coast of North Vietnam, was allegedly attacked by enemy torpedo boats. Two days later, the Maddox and another ship were fired upon by North Vietnamese guns. Within a few hours, President Johnson ordered a reprisal raid over North Vietnam.

The following day, the President sent a joint resolution to Congress seeking our approval for him to take "all necessary measures" to halt Communist aggression in Southeast Asia. The resolution passed the House in less than an hour, on a vote of 414 to 0. There was considerably more debate in the Senate, where the resolution passed 88 to 2, over the objections of Ernest Gruening of Alaska and Wayne Morse of Oregon, who insisted that the resolution was unconstitutional because only Congress can declare war.

Although I supported the President, the resolution troubled me. I couldn't prove it, but I had the feeling that the White House was using the Gulf of Tonkin incident as an excuse to open up a full-scale war. Later on it became clear that this was the case, when it was revealed that those enemy torpedo boats hadn't come within two miles of our ships.

On the morning of the vote, I was sitting as usual at John McCormack's breakfast table. The Speaker read us the text of the President's message to the House, and we discussed the resolution. During the course of the conversation, I mentioned that I was thinking of voting against it.

"Don't do it," said McCormack.

"I don't know," I replied. "It sounds to me like they were using a peashooter to bring down a tank. They didn't even hit us. The President is using this as an excuse to get us in deeper over there."

"Tom," said the Speaker, "that's not the point. Nobody is going to shoot at an American vessel on the high seas."

After breakfast, he asked me to come to his office. "If you vote against this resolution," he said, "you'll be seen as a traitor to your country. It will be the worst vote you ever make. I urge you in the strongest possible terms not to do it."

I decided to go along with his advice. But I don't want to blame John McCormack, because I was free to vote my conscience. I just didn't have the courage.

I made a big mistake that day by not voting my conscience. Politically, of course, McCormack may have been right. Had I been the only member in the House to vote against the resolution, I might have become highly unpopular among my colleagues. But that's no excuse for voting the wrong way. Since that time, I've often looked back and wondered what, if anything, would have happened differently if I had followed my instincts and opposed the President's request, which opened the door to our full-scale involvement in Vietnam.

But that's hindsight. Despite my questions about the Gulf of Tonkin Resolution, I remained loyal to Lyndon Johnson.

Not long ago, I came across a faded newspaper clipping that accurately reported my views at the time: "I feel we're in the same position today as at the time of Hitler," I said. "Had we taken a stronger stand then, there would have been no World War II. I've been led to believe that the domino theory is true. If we lose in Vietnam, we'll lose Thailand, Laos and Cambodia. Then we'd lose face in the Philippines, and Pakistan, and with those who admired our stand in Berlin."

I was aware, of course, that some people -- especially college students -- did not share my enthusiasm for the Administration's foreign policy. In fact, I was highly aware of this because my district included 22 colleges, with more students and faculty members than any other congressional district in the country. Because of the war, I was continually invited to speak at the various campuses.

Before each campus visit, I would be briefed by the White House, or Ambassador Henry Cabot Lodge, or Secretary of State Dean Rusk, or even Gen. William Westmoreland. Often the State Department or the Pentagon would send over a team to go over everything I would tell the students. Many of my colleagues were also speaking at colleges, and based on points that students had raised with them, I would be given a list of likely questions and appropriate answers.

One Friday, I was invited to speak at my alma mater, Boston College. I gave my usual talk on the war, which was followed, as usual, by a dialogue with the students. As always, they took issue with both my information and my views.

"You know," I told a young man who had challenged me, "I think I know more about this situation than you do. I've been briefed 43 times. I've been briefed by Defense Secretary Robert McNamara. I've been briefed by Gen. Westmoreland. I've been briefed by the CIA. I've been briefed by Dean Rusk. And I've been briefed by the President of the United States."

"That's a lot of briefings," said the student. "But how many times have you been briefed by the other side?"

The question came as a complete shock. Nobody had ever asked that one before.

That night, as I was lying in bed, thinking over the events of the day, I kept coming back to that student's question. And I had to acknowledge that I hadn't ever taken a good look at the other side of the issue. Before I fell asleep, I resolved to do just that.

I began my investigation of the Vietnam war during a poker game at the Army and Navy Club. One of the players was Gen. David Shoup, who had retired as commander of the Marine Corps because he couldn't go along with the Administration's policy on the war.

Shoup described his opposition to the war. "It curdles my blood to think that we're sending our boys over there on a mission we're not out to win," he said. "But that's what's going on. The will to win just isn't there. The President is afraid that if he pushes too hard, he'll start the next world war."

Shoup wasn't the only one who felt this way. Over the next few months, I met a number of experts from the Defense Department and the CIA who supported the President publicly, but who were saying just the opposite over a few beers at night. They believed that the American public was simply not prepared to have our side do what was necessary to win the war -- which was more or less to destroy North Vietnam. What really disturbed the CIA men who opposed the war was that they were having no success in conveying their views to the commander in chief. As they saw it, some of the President's top-level advisers, including Walt Rostow, John Roche, and McGeorge Bundy, were preventing their memos and reports from reaching the Oval Office.

By September, I had made up my mind that the Vietnam conflict was a civil war, and that our involvement there was wrong. But how should I make my views known? I was never one to run to the press, and even as late as 1967 I had never held a press conference. For one thing, my style has always been to operate behind the scenes. I decided that the most appropriate way to make my views known was in a newsletter to my constituents.

When I first sent out my letter on Vietnam, nobody in Washington was even aware of it. But all that changed on the night of Sept. 14, 1967.

"I guess you haven't seen the paper." My roommate, Eddie Boland, brought out the front page of the early edition of the Washington Star, which included a story with the headline "O'Neill Splits with LBJ Over Vietnam Policy." I learned later that a reporter from the Star had come across my Vietnam statement while looking through various congressional newsletters at the congressional printing office in the Cannon Building. When he realized that nobody had reported my change on the war, he decided to write it up.

According to Eddie, I was supposed to contact the White House no matter what time I got home. When I called the switchboard, the operator informed me that the President had left word that I was to come and see him at nine o'clock in the morning.

There were just the two of us in the Oval Office, and it was the first time I had seen Johnson alone since the death of Jack Kennedy. He had read the story in the paper, and he was angry and hurt at what he saw as my betrayal. "Tip, what kind of a son of a bitch are you? You, of all people! Do you think you know more about this war than I do? Do you think I don't lie in bed at night, tossing and turning over this war?"

"No, Mr. President," I replied. "But in my heart and in my conscience I believe your policy is wrong. I've talked to generals and admirals. I've talked to the CIA, and they say they can't even get their information to you. Everybody tells me you're wrong. You can't expect the country to stand behind you while you're fighting a war that can't be won."

The President calmed down. "Is that what you think?" he said. "I thought you did this for political reasons. I thought you took this position because of all the students in your area."

"No, Mr. President. The students in my area actually hurt me. My strength is not with them. It's with the back-street people. These days, my constituents cross the street when they see me coming. They don't agree with me; they agree with you. But I think this war is wrong."

I tried once more to make my point. "Mr. President, I have talked with the people who advocate your views on television and on the campuses. But most of them have real doubts. You can't win this war if you're not mining the harbors and knocking out the bridges and the power plants."

The President shook his head. "I can't do those things," he said. "They're just too dangerous. I can't risk involving the Soviets or the Chinese."

"Then let's get out of there," I said. "If we really can't win, we shouldn't be there in the first place."

The President put his arm around me. "Tip," he said, "we'll always be friends. I understand now that you're doing this because you really believe it. I'm very glad that you came in and explained yourself. As for those CIA reports you told me about, I'll make sure to see them every morning.

"Now, I want you to do me a favor. Give me time on this thing. Don't go running to the press or telling everybody your views on the war. You're the first member of the Democratic establishment to oppose me on this, and I don't want you to start the snowball rolling." He could see that the issue was ripe to explode.

I gave him my word, and we parted on good terms. In retrospect, I'm sorry I went along with his request. I wish I had talked to the press, and to my colleagues in the House. I can't say for sure that it would have made any difference, but it might have, which means it was worth trying.

Looking back, I see that Lyndon Johnson understood my power more than I did. After 14 years in the House, I was only dimly aware that I had a following. Johnson, however, knew all about it. He watched the House very closely, and he had already marked me as a leader. He understood very well that if Tip O'Neill went public, it might set off a wholesale defection.

Within a few months, however, it no longer mattered. By early 1968, Eugene McCarthy was opposing the President in the New Hampshire primary, and the stampede had already begun.

Tomorrow: Doubts about the Warren Commission. From the book "Man of the House: The Life and Political Memoirs of Speaker Tip O'Neill" by Tip O'Neill with William Novak. Copyright 1987 by Thomas P. O'Neill Jr. Reprinted by permission of the publisher, Random House Inc. Distributed by The Los Angeles Times Syndicate.